Image rejection circuits are well known in the art of radio frequency communication systems. These circuits are commonly used to minimize or eliminate the unwanted RF image of the received signal, the reception and processing of which can add noise and distortion to the communication system.
FIG. 1 illustrates a typical image rejection system employed in a communications receiver often referred to as the Harvey architecture. An RF signal 101 is provided to quadrature and in-phase mixers 104 and 106, respectively. An oscillator (not shown) supplies the in-phase mixer with an LO frequency cos(ωLOt) and the quadrature phase mixer 106 with the same LO frequency shifted 90 degrees sin(ωLOt). The quadrature and in-phase mixers 104 and 106 produces quadrature and in-phase IF signals 107 and 109, respectively. A phase shift of 90 degrees in introduced between the quadrature and in-phase IF signals 107 and 109, typically by means of a phase shifter 108. The phase-shifted quadrature and in-phase signals 111 and 113 are summed together by means of a combiner 112, which produces a combined, image-free IF signal 115.
The phase shifter 108 and the summer 112 are collectively referred to as the “IQ network” or “IQ combiner” within the art, and several different implementations currently exist. FIG. 2A illustrates a first conventional configuration consisting of an RCA (resistor-capacitor) prophase filter. In this arrangement, an RCA network provides a 90 degree phase shift between the I and Q signal paths and sums the quadrature and in-phase signals. The implementation further includes input buffer amplifiers 202 to provide the necessary impedance buffering between the mixers and the input ports of the phase shifter, and output buffer 204 coupled to the output load. The implementation suffers some disadvantageous, however, in that the resistor elements of the phase shifter degrade signal strength and noise figure. Additionally, the non-linearity and noise of the buffer amplifiers 202 and 204 further degrade the network's performance. In addition, variation in buffer gain and in the RCA components introduce gain and phase mismatch.
FIG. 2B illustrates a second conventional IQ network. This arrangement, known as the Weaver architecture, consists of second stage quadrature and in-phase mixers 222 and 224, respectively, which are supplied with a second LO frequency 90 degrees apart cos(ωLOt) and sin(ωLOt). This architecture suffers drawbacks such as complexity in its fabrication and biasing, degraded noise figure, and higher non-linearity, due to the implementation of additional mixers 222 and 224.
Accordingly, there is a need for an improved IQ network having improved performance and greater simplicity in its implementation.